The presently disclosed subject matter relates to redundant control of parallel uninterruptable power supplies (UPSs).
An uninterruptable power supply (UPS) is an electrical device that can supply power to a load despite variations in quality and/or availability of utility-based power. One common type of UPS is a double-conversion UPS. A double-conversion UPS receives power from a power source (typically a utility power grid), converts the power from alternating currents (AC) to direct current in a rectifier, and stores at least some of its power in an energy storage device. An inverter changes the DC power from the rectifier and/or the energy storage device into an AC power waveform. This AC power waveform may be supplied to the load. Several double-conversion UPSs may be connected in parallel to feed a load. At times, all of the inverters in the parallel UPS system may feed the load. During low-load conditions, some of the inverters may be switched off. Switching off some of the inverters may increase efficiency, since each inverter that is on may introduce some inefficiency.
In some installations of parallel UPS systems, an external agent may control when and how UPS modules are switched on and off via a serial connection to one of the UPS units. In other installations, the agent may be internal to the UPS system. The agent may gather information regarding system operation (including current load) and command the inverters in the parallel UPS system by switching specific UPS inverter modules online or on standby depending on load demand. This configuration posses some reliability issues, however, as it presents several single-point-of-failure conditions. Namely, a failure in the agent (e.g., software or hardware), a failure in the serial connection, and/or a failure in the UPS module receiving the serial connection could lock the system status and prevent the parallel UPS system from reacting to load steps.